1. Field of the Invention
This invention generally relates to the fabrication of liquid crystal displays and, more particularly, to a system and method of using a finned anode in the deposition of oxide films in a direct current (DC) sputtering deposition process.
2. Description of the Related Art
As noted in U.S. Pat. No. 6,149,784 (Su et al.), sputtering, or physical vapor deposition (PVD), is the favored technique for depositing materials, particularly metals and metal-based materials, in the fabrication of semiconductor integrated circuits. Sputtering has a high deposition rate and, in most cases, uses relatively simple and inexpensive fabrication equipment and relatively inexpensive material precursors, targets in the case of PVD. The usual type of sputtering used in commercial applications is DC magnetron sputtering, which is limited to the sputtering of metallic target. Sputtering is widely used for the deposition of aluminum (Al) to form metallization levels in semiconductor liquid crystal displays. More recently, copper deposition by PVD has been developed. However, sputtering is applicable to a wider range of materials useful in the fabrication of semiconductor integrated circuits. Reactive sputtering is well known in which a target of a metal, such as titanium or tantalum, is sputtered in the presence of a reactive gas in the plasma, most typically nitrogen. Thereby, the sputtered metal atoms react with the reactive gas to deposit a metal compound on the wafer, most particularly, a metal nitride, such as titanium nitride using a titanium target in a nitrogen ambient or tantalum nitride using a tantalum target in a nitrogen ambient.
FIG. 1 is a schematic block diagram, partial cross-section of a DC sputtering chamber, or reactor 100 (prior art). The reactor 100 is vacuum-sealed and has a target or cathode 102. Typically, the target 102 is a metal, but semiconductor and insulator materials can also be used. The target material is sputtered onto a substrate 104 held on a heater pedestal electrode 106 or an electrostatic chuck. An anode 108 acts as a dark space shield to protect the chamber wall 110 from the sputtered material and provides a return path or collection surface for the electrons emitted from the cathode target 102. A controllable pulsed DC power supply (not shown) negatively biases the target 102 with respect to the anode 108. Conventionally, the pedestal 106 and substrate 104 are left electrically floating, but a DC self-bias can be used to attract positively charged ions from the plasma.
Gas enters the reactor 100 from an inlet port 112, and gas exits through an exhaust port 114. The sputtering gas is often argon. The gas flow is regulated to maintain interior of the reactor 100 at a low pressure. The conventional pressure of the argon working gas is typically maintained at between about 1 and 1000 mTorr. When the argon is admitted into the reactor 100, the DC voltage applied between the target 102 and the anode 108 ignites the argon into a plasma, and the positively charged argon ions are attracted to the negatively charged target 102. The ions strike the target 102 at a substantial energy and cause target atoms or atomic clusters to be sputtered from the target 102. Some of the target particles strike the substrate 104 and are thereby deposited on it, thereby forming a film of the target material. Alternately, the target material reacts with gas added to the argon to form a composite film including target material.
To provide efficient sputtering, opposing magnets 116 and 118 produce a magnetic field within the reactor 100 in the neighborhood of the magnets 116, 118. The magnetic field traps electrons and, for charge neutrality, the ion density also increases to form a high-density plasma region 120 within the reactor adjacent to the target 102.
Plasma ignition can present a significant problem, especially in the geometries representative of a commercially significant plasma reactor. The initial excitation of a plasma requires a high voltage, though with essentially no current, to cause the working gas to be excited into the electrons and positive ions of an electron. This condition must persist for a time period and over a space sufficient to support a low-resistance, essentially neutral plasma between the two electrodes in the case of a capacitively coupled plasma. The maintenance of a plasma requires a feedback condition in which argon atoms must supply as many electrons to the anode as ions to target. If the flow of electrons to the anode is insufficient, the plasma collapses or is never formed.
Pulsed DC sputtering also provides a method for the low temperature (less than 2000 degrees C) deposition of oxide films, and should have advantages over the current plasma-enhanced chemical vapor deposition (PECVD) process. Low temperature processing is a critical when films are deposited on plastic substrates, such as the substrates used in the fabrication of flexible liquid crystal displays (LCDs). In addition, the pulsed DC sputtering of SiO2 and SiNx shows good promise from the viewpoint of high deposition rates and process flexibility. For example, the composition of the deposited film can be changed by simply changing gas mixture. However, the quality and deposition rate oxide films formed by pulsed DC sputtering is highly dependent on maintaining a good conductive anode for the electron return path.
FIG. 2 is a schematic block diagram of the chamber 100 of FIG. 1 after the processing of a few substrates (prior art). The same oxide film that is being deposited on the substrate 104 is also being deposited on the anode 108. Once the anode 108 is covered with oxide, a highly resistive material, the electron return path through the anode is eliminated. In response to the anode 108 being covered by the insulating film, changes occur in the plasma condition capacitance and electron flow in the chamber 100. As the oxide layer on the anode 108 increases, electrons charge the anode surface, which produces a large capacitance the chamber. The large capacitance creates micro arcing as the capacitance builds ups and discharges. When the anode is heavily coated, the capacitance in the chamber becomes very large and severe arcing will occur along with damage to the target 102.
Micro arcing is the first symptom that the anode is covered with insulator. Areas 200 of the substrate are damaged as a result of micro arcing. The resulting film can be nonuniform and of a poor quality. As the barrier layer on the anode increases in thickness, more severe arcing will take place in the vacuum chamber and eventually loss of plasma will occur. Severe arcing can cause damage. to the target 102 and result in the formation of large particles 202 on the substrate 104. Such damage creates manufacturing problems, such as a short anode life cycle, poor film quality, and low production yield, all a result of arcing in the chamber. Because arcing aggravates film quality and production yield, conventional design anodes must be frequently changed and cleaned. These frequent anode changes are detrimental to production efficiency.
FIG. 3 is a schematic block diagram of the chamber of FIG. 1 using an anode 108 having slits 300 on the anode surface (prior art). Alternately stated, the anode 108 has ribs 302 between the slits 300. The principle behind this modification is to increase the aspect ratio of the anode surface and, therefore, increase anode life for manufacturing. Although the vertical section of the ribs appear to collect less deposition material than the horizontal surfaces, the overall ratio between cathode and anode still increases as the anode gets coated. As the ratio between the cathode and anode changes, so does the chamber capacitance and plasma. In other words, effective area of anode decreases during the process and causes changes to the plasma distribution and film uniformity. After the insulating oxide film covers the anode, arcing occurs as described above.
It would be advantageous if electrical insulator material, such as oxide, could be deposited efficiently using a DC sputtering process.
It would be advantageous if the same anode in a DC magnetron could maintained in place through many substrate deposition cycles for greater manufacturing efficiency.
It would be advantageous if the electron collection surface of an anode could be kept free of deposition material during the DC sputtering of electrical insulator films.
The present invention relates to an improvement in the DC sputtering of oxide films on a substrate. This invention reduces the effects of the xe2x80x9cdisappearing anodexe2x80x9d during DC sputtering of oxides on glass or other insulators. More specifically, a new anode concept is presented. The present invention anode provides a good electron return path, while maintaining the necessary aspect ratio between the cathode and the anode for longer periods of time. This new design anode incorporates an anode fin with holes in the surface of the fin. The fin shields a section of the anode from the oxide film to provide the return path for the electrons.
Accordingly, in a liquid crystal display (LCD) fabrication process including a pulsed DC sputtering system with a cathode and an anode, method is provided for depositing films. The method comprises: supplying a substrate having a horizontal top surface; supplying a target cathode of a first material; supplying an anode having an electron collection surface; shielding the electron collection surface of the anode from the deposition of a second material; and, sputter depositing the substrate top surface to form a second material film.
Shielding an electron collection surface of an anode from the target first material includes: forming an anode with vertical and horizontal ribs, and a fin having a top surface and a bottom surface to collect electrons, interposed between the anode ribs. Then, the method comprises: selecting the fin first angle, measured with respect to the substrate top surface. The exact fin angle is dependent upon factors such as the chamber size and other process specifics.
In some aspects of the invention, shielding an electron collection surface of an anode from the deposited second material includes forming vias from the fin top surface to the fin bottom surface to induce the collection of electrons on the fin top surface, through the vias. In other aspects, shielding an electron collection surface of an anode from the deposition of the second material includes magnetically defecting the flow of electrons to the electron collection surface.
Additional details of the above-described method, and a pulsed DC sputtering system for depositing films, are provided below.